Finite Populations & Finite Time: The Non-Gaussianity of a Gravitational Wave Background

TL;DR

This paper investigates how finite populations of supermassive black hole binaries cause non-Gaussian features in gravitational wave background signals detected by pulsar timing arrays, challenging the Gaussian assumption in current models.

Contribution

The authors develop analytical and numerical models of the gravitational wave background from SMBHBs without common simplifying assumptions, revealing non-Gaussian effects.

Findings

Finite populations induce non-Gaussianities in PTA signals.

Windowing effects contribute to non-Gaussian features.

Current PTA analyses do not account for these non-Gaussianities.

Abstract

Strong evidence for an isotropic, Gaussian gravitational wave background (GWB) has been found by multiple pulsar timing arrays (PTAs). The GWB is expected to be sourced by a finite population of supermassive black hole binaries (SMBHBs) emitting in the PTA sensitivity band, and astrophysical inference of PTA data sets suggests a GWB signal that is at the higher end of GWB spectral amplitude estimates. However, current inference analyses make simplifying assumptions, such as modeling the GWB as Gaussian, assuming that all SMBHBs only emit at frequencies that are integer multiples of the total observing time, and ignoring the interference between the signals of different SMBHBs. In this paper, we build analytical and numerical models of an astrophysical GWB from circular, inspiralling binaries inclined relative to the line-of-sight of the observer, without the above approximations, and compare the statistical properties of its induced PTA signal to those of a signal produced by a Gaussian GWB. We show that finite population and windowing effects introduce non-Gaussianities in the PTA signal, which are currently unmodeled in PTA analyses.